It can be very difficult and expensive to monitor vacuum below 1×10−9 Torr (i.e. the pressure regimen in photoinjector and other high vacuum systems used throughout the world). Vacuum gauges designed to operate at this pressure range, such as extractor gauges and residual gas analyzers, cost thousands of dollars. At the Jefferson National Laboratories, for example, where vacuum systems can be very large (i.e. the entire length of a 10 meter photoinjector accelerator beam line), it is cost prohibitive to install enough gauges to accurately monitor the vacuum quality throughout the entire system.
Besides the expensive involved in such installations, these types of vacuum gauges suffer other shortcomings. The gauges contain hot filaments that produce gas within the vacuum system. It is counterproductive to use a vacuum gauge that inherently degrades vacuum while in use. The gauges also produce light, that can generate unwanted photoemission from photocathodes within the polarized electron sources used in such installations.
Ion pumps are commonly and extensively used throughout the accelerator facilities of the Jefferson National Laboratory and at other government and commercial institutions to generate ultra high vacuums. Ion pumps contain metal plates that are biased at high voltage. Gas within the vacuum chamber is ionized as it passes between the ion pump plates (i.e. electrons are stripped from the gas atoms and molecules). Once ionized, the gas atoms and molecules embed themselves within the metal plates of the ion pump. In this manner, the gas within the vacuum chamber is “pumped” away, i.e. removed from the vacuum chamber.
The ionized gas that becomes embedded within the plates of the ion pump constitutes an electrical current. This electrical current provides a measure of the vacuum quality within the vacuum chamber. At high pressure, ion pumps produce relatively large electrical current (milliamperes) that is easily measured. At lower pressures, ion pumps produce less current. At extremely low pressure, commonly referred to as “ultra high vacuum”, it can be difficult to measure the small electrical current that is produced by an ion pump.
The manufacturers of ion pumps provide a measurement of the electrical current produced by an ion pump. Unfortunately, this value is limited to ˜1×10−6 amperes, which corresponds to 1×10−9 Torr vacuum pressure. Many ultra high vacuum systems operate below this value.
Thus, there exists a need for a coat effective system and method for monitoring ultra high vacuum environments such as those found in many institutional government and industrial installations.